A relation between the extent of DNA crosslinking and its alkaline elution properties has been established in L1210 cells in culture exposed to nitrogen mustard. This reaction allows a quantitative study of in vivo L1210 DNA crosslinking in tumor bearing mice exposed to the synergistic antitumor combination of Adriamycin and cyclophosphamide. The results indicate that a reduced ability to remove interstrand DNA crosslinks in cells exposed to the combination is responsible for the synergistic response.